A drive control apparatus for a vehicle designed to improve vehicle maneuverability by adjusting a drive torque of a drive source in accordance with a turning state of a vehicle is known from, for example, Japanese Patent Application Laid-Open Publication No. 09-86378 (JP 09-86378 A).
The vehicular drive control apparatus of JP 09-86378 A calculates the difference between a target yaw rate and an actually occurring yaw rate (yaw rate deviation), and when the yaw rate deviation exceeds a prescribed threshold, the apparatus calculates a transfer torque, and transfers the torque between the left and right rear wheels in an amount commensurate with the transfer torque. As a result, vehicle maneuverability improves because a yaw moment is generated in the vehicle.
The scheme for transferring torque to the left and right in this way while the vehicle is turning (a so-called scheme for adding turn assistance), may be a scheme for transferring torque between the left and right rear wheels, a scheme for transferring torque between the left and right front wheels, or a scheme for transferring torque between the left and right front and rear wheels.
The turning characteristics of a vehicle in the following usage examples will be discussed for a case in which the drive control system for a vehicle of the prior art is adopted in a general four-wheel-drive vehicle (a vehicle type in which the left and right front wheels and the left and right rear wheels are driven by using a drive source).
The first usage example is one in which only the front left and right wheels are driven and the rear wheels are not driven, i.e., there is no added turn assist from the rear wheels. Front wheel drive torque (forward drive torque) refers to drive torque for the front wheels.
The orientation of the wheels in relation to the forward direction of the vehicle shifts by a slip angle when a vehicle has been turned. Accordingly, the drive torque of the drive source is not directly transmitted from the front wheels to the road surface. As a result, the vehicle speed during a turn is reduced with respect to the speed of a vehicle traveling straight forward.
The second usage example is one in which the front right and left wheels are driven together with added turning assist from the rear wheels. The term “total drive torque” will be referred to as a drive torque for driving all the wheels. The total drive torque of the second usage example may be set to be the same as the drive torque for the front wheels (forward drive torque) of the first usage example.
Vehicle maneuverability is improved in the second usage example because torque is transferred between the left and right rear wheels in the manner of the drive control apparatus of JP 09-86378 A. When maneuverability is improved, the steering angle of the steering wheel may be small (vehicle handling is good) in comparison with the first usage example. The smaller the steering angle is, the smaller the wheel slip angle is in relation to the forward direction of the vehicle. Accordingly, the drive torque of the drive source readily transfers from the wheels to the road surface. As a result, the vehicle speed is not substantially reduced during a turn in relation to the vehicle speed during forward travel. Acceleration is even possible depending on the suspension setting.
In general, the greater the driving experience of the driver, the more likely it is that the driver will have as an empirical rule the relationship between the steering angle and the reduction in vehicle speed when a change is made from forward travel to turning. Accordingly, it is preferable that the vehicle speed during a turn should be reduced to an extent that the driver does not have an unpleasant sensation. Since the vehicle speed is not reduced during a turn as in the first usage example, there is room for improvement in increasing the driving energy efficiency during a turn.